In marine seismic exploration an acoustic source may be used to produce an acoustic signal. Typically, a number of airguns are deployed. Each airgun produces a pressure wave whose signature is dependent on a number of factors, including the volume of air released, the original pressure of air within the airgun and the hydrostatic pressure of the surrounding water. In addition, the water depth has an effect since a wave travelling up from the airgun may be reflected at the water/air interface at the surface.
The array produces a pressure wave which is a combination of each individual gun element. A number of sensors detect reflections from within the earth's crust to produce a dataset for analysis.
A significant amount of processing is required to interpret and clarify the data. Since the dataset is a time series of reflections of the original array signature, a signature deconvolution is used to simplify the data signal. This deconvolution requires the source signal to be known. Such source signatures may be derived in advance or measured.
For the highest quality data, it is important both that the source signature is as close as possible to the design signature and be achieved consistently from shot to shot.
Airguns are commercially available, and typically have two chambers containing compressed air. The larger chamber contains the main air volume which is to be released into the water to produce the pressure wave which is the airgun's purpose. This document will refer to this chamber as ‘the firing chamber’.
A second, smaller chamber, having an electrical solenoid valve control system, is used to cause a pressure imbalance between the two chambers and cause a ‘shuttle’ or ‘sleeve’ to move, releasing the air from the firing chamber into the surrounding water. This chamber will be referred to as ‘the control chamber’ in this document.
An airline may be used to fill the chambers and an electrical sensor used to detect the exact firing time of the airgun. US 2007/0263489 describes such an airgun.
To achieve repeatable results, it is important that the individual sources (the airguns which combine together to produce the ‘array’) need to be accurately controlled. It is in particular important that each airgun fires at the same time, and only at that time. Each gun needs to be at the design depth, with the design volume of air at the design pressure.
To this end, a source synchronizer may be used which uses the signals from the detection sensor to adjust the firing of individual airguns in a feedback loop. The depth of each sensor may be measured by depth transducers included in the array. The pressure may be measured by pressure transducers either attached to the airline or indeed in a vessel towing the array.
In spite of much work in developing reliable airguns and repeatable measurements, a number of problem areas exist.
Firstly, air leaks may occur in the supply system. Large air leaks may of course be detected by pressure sensors which may be placed within the air supply to the gun. Small air leaks may however go unnoticed in particular because of the large volume of air used to supply the airguns. Small leaks can however give rise to aerated water in the proximity of the airguns which affects the surface reflected pressure wave propagation, and also leads to firing airguns at considerably lower pressure than is being measured by the pressure sensors in the system.
Thus, such small air leaks can be undetectable by usual means but still cause considerable data degradation.
A second problem that may occur is faulty electrical connections. The electrical connections to airguns are in a challenging environment, being towed underwater and further being subject to the shock from the pressure wave being produced on activation of the airguns. They may suffer from inductive or capacitive pick-up from neighbouring sensor, solenoid or auxiliary cabling, leakage to earth or to neighbouring connectors, intermittent connector or short circuits, or signal corruption.
The problem is made more difficult by the vast amount of data that is commonly collected in such systems. It is very difficult, if not impossible, to monitor all the data generated for problems by human observation.
The detection of airgun faults is accordingly important. GB 2 445 842 teaches comparing a measured near field signal measured on a near field transducer with a reference near field signal and checking if the difference exceeds a threshold.
Another approach is provided by GB 2 438 080 which discloses the addition of a pressure sensor into the firing chamber to measure the pressure during discharge of the airgun. However, the addition of such an additional sensor into the charge chamber of an airgun is very difficult to achieve reliably, bearing in mind that the firing chamber of such an airgun is an explosive device. The introduction of any hole or sensor into such a charge chamber can greatly reduce the strength and hence reliability, and the difficulties and expense of introducing an electrical connection and cabling in this harsh environment is at least problematical.
Similar approaches, which also introduce a pressure sensor into the charge chamber are provided by GB 2 394 046 and FR 2 762 398.
A secondary device often used to monitor airgun performance is the near-field hydrophone. This will be placed in the proximity of the airgun, but at such a distance as to be outside the initial air-bubble caused on air expulsion. This distance will be typically one meter from the airgun ports. As the hydrophone is actually measuring the pressure wave from the airgun, this should be the most definitive method to monitor it's performance.
Calculating faults based on near field sensors, i.e. hydrophones separate from the marine acoustic sources, has been proposed in GB2445842 but this requires both a separate near-field sensor (the hydrophone) and is also computationally rather difficult.
There are significant problems in using these devices successfully in a production environment, not least being the physical reliability of the hydrophone itself. The near field sensor is typically a piezo-electric sensor which is subject to damage if it is crystalline, and to desensitisation if it is of piezo-electric plastic materials such as pdvf (Polyvinylidene Fluoride). Further the pressure it is measuring is not only caused by the target airgun, but other airguns in proximity, which may be just as close to the hydrophone, and the reflections from the sea surface, which will vary with sea state and surface wave motion.
Reliability issues such as those described above exist not just with airguns, which are most widely used, but with alternative marine acoustic sources such as waterguns.